a) Field of the Invention
This invention relates to an aqueous pigment dispersion, and especially to an aqueous pigment dispersion useful in printing, recording, drawing, stamping or otherwise presenting characters, patterns or pictures (hereinafter collectively called "marks") on paper, films or sheets by writing instruments, recording equipments or the like which make use of capillary phenomena. The present invention is also concerned with a water-soluble resin useful for the preparation of the aqueous pigment dispersion, a production process of the water-soluble resin, and an equipment suitable for use with the aqueous pigment dispersion.
b) Description of the Related Art
Aqueous pigment dispersions--each of which makes use of an acrylic resin, which contains carboxyl groups as solubilizing groups, as a pigment-dispersing resin--have conventionally been used as water-based pigment inks for plastic-fiber-tip pens, felt-tip pens, inkjet printers and the like for their high dispersion stability and also owing to good waterproofness and durability of written or recorded marks.
Marks of such a water-based pigment ink are superior in fastness such as waterproofness and light resistance to marks of a dye-based ink, but in terms of the vividness and density of a color of marks or the gloss of marks written or recorded on a non-absorptive surface, the former are inferior to the latter. These drawbacks can be attributed to the use of capillary phenomena by a writing instrument or recording equipment in which such a water-based pigment ink is used. To permit the use of capillary phenomena, the ink is required to have low viscosity. To provide the ink with low viscosity, the ink is prepared by using a pigment-dispersing resin in a smaller amount.
A water-based pigment ink desired for such applications is supposed to have high density and low viscosity and, when written or recorded on a liquid-absorptive surface, also to exhibit high surface tension. Moreover, the dispersion of the pigment in the water-based pigment ink does not change and remains stable with time, and written or recorded marks have a vivid color and are excellent in waterproofness, light resistance and other durability.
For the vividness of a color of marks and the settling stability of a pigment in a water-based pigment ink (i.e., the resistance of the pigment to settling during storage), the pigment is desirably in the form of fine particles. To achieve uniform dispersion of such fine pigment particles in an aqueous medium, use of a pigment-dispersing resin is needed. Concerning the ratio of the pigment-dispersing resin to the pigment to be used, there is an optimal value in view of the dispersibility, dispersion stability and color-developing ability of the pigment. Although this optimal value also varies depending on the type of the pigment, the pigment-dispersing resin has to be used in a greater amount as the particle size of the pigment becomes smaller.
To obtain a water-based pigment ink of high pigment concentration and low viscosity by using a fine particulate pigment, the molecular weight of a pigment-dispersing resin must therefore be lowered. However, a certain higher molecular weight is desired for a pigment-dispersing resin from the standpoint of dispersion stability of a pigment with time.
Carbon black has been used as a black pigment for many years, and features excellent density and durability as its advantages. Due to its extremely small particle size, however, the molecular weight of a pigment-dispersing resin must be lowered in order to fully disperse it in an aqueous medium. Nevertheless, conventionally-known, pigment-dispersing resins are each accompanied by a problem that the dispersion stability of a pigment is reduced if the molecular weight is lowered.
Further, a condensation product between sodium naphthalenesulfonate and formaldehyde, which has conventionally been used as a dispersant for pigments, is excellent in pigment dispersibility and the dispersion stability of pigment dispersions and permits preparation of an aqueous pigment dispersion of high concentration. The condensation product is however accompanied by a problem that written or recorded marks are poor in waterproofness and also a problem that good writing comfort cannot be enjoyed due to excessively sluggish flow property of the dispersion. Mere writing ability is not sufficient for a writing instrument. It is important for a writing instrument to permit writing with feeling which is in conformity with the user's desire or taste. Writing comfort is therefore an important property for writing instruments.
Keeping in step with the recent advancements in ink-jet printing and jet printing machines and the recent popularization of on-demand printing, there is an ever-increasing demand for the development of a water-based pigment ink which has high pigment concentration and low viscosity and has overcome the above-described problems of the conventional art.